The sensitivity of leakage to pressure has been investigated for a number of decades, but using a scientifically inferior power equation formulation (called the N1 power leakage equation). This is an empirical equation that doesn't describe the observed results in terms of their underlying causes, but is still used almost exclusively in water loss research and practice.
Pressure management has several proven benefits, including reduced leakage rates, lower rates of pipe bursts, longer infrastructure service life and more efficient water consumption. In pressure management, a pressure reducing valve (PRV) is commonly installed at the inlet or inlets of a district metered area to reduce the water pressure in the district metered area. Pressure management is especially useful in district metered areas where the pressure rises substantially above the minimum value required for adequate service provision. Pressure management has several proven benefits, including reduced leakage rates, lower rates of pipe bursts, longer infrastructure service life and more efficient water consumption.
Real water losses (or real losses) are defined as the water that is physically lost from the water distribution system through leak openings in pipes and other distribution network components. Water consumption from distribution systems varies diurnally, with the minimum consumption typically occurring in the early morning hours. The inflow into a district metered area during the lowest consumption period is called the minimum night flow (MNF), typically between 02 h00 and 04 h00. The real losses in a district metered area are estimated by subtracting the estimated user consumption from the minimum night flow.
The flow rate through a leak opening in a pipe or network component is a function of the fluid pressure as described by the orifice equation, which is derived from the principle of conservation of energy in fluids. The orifice equation is as follows:Q=CdA√{square root over (2gh)}  (1)where Q is the flow rate through the leak; Cd is the discharge coefficient; A is the leak area; and h is the fluid pressure head at the leak.
The discharge coefficient Cd in Equation 1 is used to compensate for energy losses and local effects that occur at leaks. Several studies have been done to estimate the discharge coefficient for different types of openings, and a reasonable estimate can thus normally be made.
In spite of the fact that the idea of a linear relationship between pressure and leak area has been assumed by others in the past (most notably by May in 1994, but by some others long before him), the idea has not found any acceptance in practice of which applicant is aware. Whilst John May and Allan Lambert developed the fixed and variable area discharge (FAVAD) concept that relies on the linear assumption in the 1990s, they did not recommend the use of the linear relationship in practice. Instead they continued to support practitioners in using the conventional N1 power equation which is a simple empirical equation with no scientific basis. As a result, people of ordinary skill in the art invariably use the N1 equation for their analyses and predictions.
Mark Shepherd and Allan Lambert (widely considered to be the doyen of leakage management in the world) are also proponents of the N1 equation and resist applying a linear relationship concept in practice. This indicates a very strong empirical tradition that has become embedded within the leakage management industry in that people feel that the N1 concept works adequately and thus they resist suggestions to move away from it.
Applicant is not aware of any attempts to apply the idea of linear pressure-area to characterise district metered area (DMA) leakage at a practical level.
The areas of leaks are not fixed, but vary with fluid pressure. It has been established through several studies that the leak area is generally a linear function of pressure:A=A0+mh  (2)where A is the area of the leak at a given pressure h; A0 is the initial leakage area, defined as the leak area under zero pressure conditions; and m is the head-area slope, defined as the rate at which the leak area expands with increasing pressure. The initial leakage area and head-area slope are collectively referred to as the leakage parameters.
Since the discharge coefficient is not generally known in advance, a useful approach is to lump it with the area A in Equations 1 and 2 and head-area slope m in Equation 2, with CdA called the effective leakage area and Cdm the effective head-area slope.
Irrespective of the foregoing, the occurrence of new leaks in a district metered area (DMA) has not been adequately considered or addressed.
The preceding discussion of the background to the invention is intended only to facilitate an understanding of the present invention. It should be appreciated that the discussion is not an acknowledgment or admission that any of the material referred to was part of the common general knowledge in the art as at the priority date of the application.
In this specification the term minimum night flow is intended to receive a realistic interpretation as it would be understood by one of ordinary skill in the art and is the flow taken at a time when the consumption is at or near its lowest during a normal 24 hour period, but not necessarily, between the hours of 02 h00 and 04 h00, so that losses through leakage are at or near a high value in comparison to consumption within the district metered area.
In this specification the term ‘leakage characteristics’ refers to two properties of a leak or a group of leaks, namely its initial area and head-area slope. The initial area is the size of the leak opening under zero pressure conditions and the head-area slope is the rate at which the leak area varies in size with changes in pressure.